1. Field of the Invention
This disclosure relates to servicing a wellbore. More specifically, it relates to servicing a wellbore with compositions comprising a low heat of hydration cement.
2. Background of the Invention
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
The completion of subterranean wellbores in fragile geographic zones such as in permafrost poses particular challenges. Permafrost is defined as soil that stays in a frozen state for more than two years. Cement compositions for use for in subterranean formations within zones of permafrost must be designed to set before freezing and have a low heat of hydration. Large amounts of heat evolved, for example during the hydration of a hydraulic cement result in temperature increases that may be sufficient to destabilize the formation.
In addition to destabilizing the formation, high heats of hydration promote the evolution of gas from gas hydrates (e.g. methane hydrate) that are present in large amounts, typically below the sea floor in regions that slope from the continents to the deep ocean basins. They also occur in Arctic regions where permafrost formations exist. Gas hydrates are a solid ice-like phase formed at low temperature and high pressure by van der Waals forces between gas and water molecules, with the “host” water molecules forming a molecular cage which confines the “guest” gas molecules through their mutual electrostatic interaction. Most low molecular weight gases (O2, N2, CO2, CH4, H2S, Ar, Kr, Xe etc.) will form a hydrate under some pressure-temperature conditions. In the ocean, gas hydrates composed predominantly of methane are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Gas hydrates are believed to form by migration of gas from depth along faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. During well construction operations, if the temperature of the formation is raised above the temperature at which the gas is released from the gas hydrates, the released gas may destabilize the wellbore or form flow channels in and around the set cement and lead to loss of effective zonal isolation.
Compositions for use in subterranean formations comprising permafrost and/or gas hydrates must provide certain properties for a given use while minimizing the impact of these compositions on the stability of the formation, and thus an ongoing need exists for such compositions.